Connecting rods are used in engines of, for example, automobiles. The connecting rod couples a piston to a crankshaft to convert the vertical motion of the piston to the rotational motion of the crankshaft.
FIG. 1 is a front view of a conventional connecting rod 1. As illustrated in FIG. 1, the conventional connecting rod 1 includes a big end portion 10, a rod portion 20, and a small end portion 30. The big end portion 10 is disposed at one end of the rod portion 20 and the small end portion 30 is disposed at the other end of the rod portion 20. The big end portion 10 is coupled to a crank pin. The small end portion 30 is coupled to a piston.
The conventional connecting rod 1 includes two parts (a cap 40 and a rod 50). The cap 40 and one end of the rod 50 correspond to the big end portion 10. The other portions than the one end of the rod 50 correspond to the rod portion 20 and the small end portion 30.
The big end portion 10 and the small end portion 30 are formed by machining. Thus, the connecting rod 1 needs to exhibit high machinability.
Furthermore, during operation of the engine, the connecting rod 1 is subjected to loading from nearby components. Furthermore, for fuel saving, there have been needs in recent years for size reduction of the connecting rod 1 and an increase in cylinder pressure within the cylinder. Accordingly, there is a need for the connecting rod 1 to have a thinner rod portion 20 and at the same time be able to exhibit high buckling strength sufficient to withstand the explosive loading transmitted from the piston. The buckling strength heavily depends on the yield strength of the material. Thus, connecting rods need to exhibit high yield strength as well as high machinability.
In the conventional connecting rod 1, the cap 40 and the rod 50 are separately produced as described above. Thus, for positioning of the cap 40 and the rod 50, a dowel pinning process is performed. Furthermore, a machining process is applied to the mating surfaces of the cap 40 and the rod 50. In view of this, fracture splitting connecting rods, which make it possible to eliminate these processes, are increasingly being employed.
A fracture splitting connecting rod is formed by forming a one-piece connecting rod and then fracturing the big end portion thereof into two parts (corresponding to the cap 40 and the rod 50). When mounting it to an engine, the split two parts are joined together. Thus, the dowel pinning process and the machining process are not performed. This results in reduced production cost.
Technologies relating to a steel material for such a fracture splitting connecting rod and a method for producing such a fracture splitting connecting rod are disclosed in U.S. Pat. No. 5,135,587 (Patent Literature 1), Japanese Patent Application Publication No. 2010-180473 (Patent Literature 2), Japanese Patent Application Publication No. 2004-301324 (Patent Literature 3), International Application Publication No. WO 2012/164710 (Patent Literature 4), Japanese Patent Application Publication No. 2011-084767 (Patent Literature 5), and International Application Publication No. WO 2012/157455 (Patent Literature 6).
Patent Literature 1 discloses the following. A steel for fracture splitting connecting rods contains, in weight %, C: 0.6 to 0.75%, Mn: 0.25 to 0.50%, and S: 0.04 to 0.12%, the balance being Fe and up to 1.2% of impurities. Mn/S is 3.0 or more. The steel has a 100% pearlitic structure and a grain size of 3 to 8 ASTM per Specification E112-88.
Patent Literature 2 discloses the following. A steel for fracture splitting connecting rods is a non-heat treated steel made up of ferrite and pearlite and containing 0.20 to 0.60% of C in mass %. The rod portion is subjected to a coining process. The steel for fracture splitting connecting rods contains C, N, Ti, Mn, and Cr as essential elements and contains Si, P, S, V, Pb, Te, Ca, and Bi as optional elements. The essential elements include, in mass %, 0.30 to 1.50% of Mn, 0.05 to 1.00% of Cr, 0.005 to 0.030% of N, and 0.20% or less of Ti. The formula, Ti≥3.4N+0.02, is satisfied. The 0.2% proof stress of the big end portion is lower than 650 MPa. Further, the 0.2% proof stress of the rod portion, which has been subjected to the coining process, is higher than 700 MPa.
Patent Literature 3 discloses the following. A non-heat treated connecting rod contains, in mass %, C: 0.25 to 0.35%, Si: 0.50 to 0.70%, Mn: 0.60 to 0.90%, P: 0.040 to 0.070%, S: 0.040 to 0.130%, Cr: 0.10 to 0.20%, V: 0.15 to 0.20%, Ti: 0.15 to 0.20%, and N: 0.002 to 0.020%, the balance being Fe and impurities. The Ceq value defined by Formula (1) is less than 0.80. The structure of the big end portion is made up of ferrite and pearlite. The total hardness of the big end portion ranges from 255 to 320 on the Vickers hardness scale. Further, the hardness of the ferrite of the big end portion is 250 or more on the Vickers hardness scale. Further, the hardness of the ferrite relative to the total hardness of the big end portion is 0.80 or more.Ceq=C+(Si/10)+(Mn/5)+(5Cr/22)+1.65V−(5S/7)  (1)
Patent Literature 4 discloses the following. A non-heat treated steel bar for connecting rods contains, in mass %, C: 0.25 to 0.35%, Si: 0.40 to 0.70%, Mn: more than 0.65% to 0.90% or less, P: 0.040 to 0.070%, S: 0.040 to 0.130%, Cr: 0.10 to 0.30%, Cu: 0.05 to 0.40%, Ni: 0.05 to 0.30%, Mo: 0.01 to 0.15%, V: 0.12 to 0.20%, Ti: more than 0.150 to 0.200% or less, Al: 0.002 to 0.100%, and N: 0.020 or less, the balance being Fe and impurities. Fn1, defined by the formula below, ranges from 0.60 to 0.80, and Fn2, defined by the formula below is 7 or more. In the structure of the non-heat treated connecting rod steel, the ferrite and pearlite structure accounts for 90% or more. The proportion of the ferrite in the ferrite and pearlite structure is 40% or more.Fn1=C(Si/10)+(Mn/5)+(5Cr/22)+1.65V−(5S/7)+(Cu/33)+(Ni/20)+(Mo/10)Fn2=(Mn Ti)/S
Patent Literature 5 discloses the following. A method for producing a fracture splitting connecting rod includes: a step of providing a steel material; a step of heating the steel material to a temperature ranging from 1200° C. to 1300° C.; a step of hot forging the steel material into a rough forged body, the step being carried out by applying compression to the steel material at at least a predetermined portion thereof at a temperature of 1000° C. or more and at a working ratio of 50% or more; and a step of cooling the rough forged body at at least 5° C./s or less to form a ferrite and pearlite structure therein. The resulting fracture splitting connecting rod contains, in mass %, C: 0.16 to 0.35%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.0%, P: 0.040 to 0.070%, S: 0.080 to 0.130%, V: 0.10 to 0.35%, and Ti: 0.08 to 0.20%. The hardness of the predetermined portion is at least 250 HV or more.
Further, Patent Literature 6 discloses a non-heat treated steel having a low V content. Specifically, Patent Literature 6 discloses the following. The non-heat treated steel contains, in mass %, C: 0.27 to 0.40%, Si: 0.15 to 0.70%, Mn: 0.55 to 1.50%, P: 0.010 to 0.070%, S: 0.05 to 0.15%, Cr: 0.10 to 0.60%, V: 0.030% or more to less than 0.150%, Ti: more than 0.100% to 0.200% or less, Al: 0.002 to 0.050%, and N: 0.002 to 0.020%, the balance being Fe and impurities. Et, defined by the formula below, is less than 0. Ceq, defined by the formula below, is more than 0.60 to less than 0.80.Et=[Ti]−3.4[N]−1.5 [S]Ceq=[C]+([Si]/10)+([Mn]/5)+(5 [Cr]/22)+(33 [V]/20)−(5 [S]/7)
The steel for fracture splitting connecting rods of Patent Literature 1 has been widely commercialized in Europe. However, the steel for fracture splitting connecting rods of Patent Literature 1 may have low yield strength and machinability in some cases.
The steel for fracture splitting connecting rods disclosed in Patent Literature 2 has high yield strength. However, it may have low fracture splittability in some cases.
Furthermore, production conditions for hot forging, e.g., the heating temperature prior to hot forging, may vary from production site to production site. If a fracture splitting connecting rod is produced using any of the steel materials and the production methods disclosed in Patent Literatures 1 to 6 with the heating temperatures prior to hot forging being non-uniform, the fracture splitting connecting rod, in some cases, has a low fracture splittability, low yield strength, or low machinability.